Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.
2 min synopsisUK scopeSources checked 13 Sept 2026Clinical review pending
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Obstructive hydrocephalus or brainstem compression
A posterior-fossa lesion can obstruct the fourth ventricle or compress the brainstem in a confined compartment. Drowsiness, repeated vomiting, new cranial-nerve dysfunction, irregular breathing, bradycardia or rapidly worsening gait may precede herniation.
Action: Escalate immediately to paediatric or adult neurosurgery as appropriate, support airway and circulation, perform serial consciousness, pupil and brainstem observations, obtain urgent CT when unstable and definitive contrast MRI when safe; do not perform lumbar puncture in suspected mass effect.
Synopsis
Recognise posterior-fossa localisation and obstructive hydrocephalus, investigate safely, and distinguish adult and paediatric tumour patterns without transferring one population’s treatment rules to the other.
The posterior fossa contains cerebellum, brainstem, fourth ventricle and lower cranial nerves; little spare volume means a modest lesion can cause early CSF obstruction or brainstem compromise.
Presentation combines cerebellar signs, cranial-nerve dysfunction and raised-pressure features: gait or truncal ataxia, dysmetria, nystagmus, dysarthria, diplopia, vomiting and headache.
Age changes probability and treatment: childhood medulloblastoma, pilocytic astrocytoma and ependymoma are important entities, whereas adults more often have metastasis and extra-axial or cranial-nerve tumours, but imaging and tissue establish diagnosis.
Key red flags
Declining consciousness, abnormal pupils, respiratory irregularity or rapidly progressive weakness with a posterior-fossa lesion indicates a pressure emergency.
Repeated or early-morning vomiting, worsening headache, papilloedema or impaired upgaze can reflect obstructive hydrocephalus rather than a primary gastrointestinal disorder.
New truncal or gait ataxia, dysmetria, nystagmus, dysarthria or cranial-nerve palsy localises to cerebellar or brainstem networks and requires timely imaging.
A child with newly abnormal cerebellar or other central neurological function follows a very urgent paediatric assessment pathway; adult tumour epidemiology and treatment must not be copied across.
Sudden severe headache or abrupt deficit raises haemorrhage, infarction or acute CSF obstruction in addition to tumour progression.
Neck stiffness with headache and vomiting can occur with posterior-fossa pathology but also infection or subarachnoid haemorrhage; lumbar puncture is unsafe until mass effect has been assessed.
Hydrocephalus and pressure
Ask about progressive headache, repeated vomiting, drowsiness, transient visual obscurations and diplopia; examine consciousness, pupils, fundi and upgaze. Rapid change requires emergency imaging even if headache is not dramatic.
Brainstem and cranial nerves
Assess eye movements, facial sensation and movement, hearing, palate, voice, tongue, swallowing and limb reflexes. Multiple cranial neuropathies with long-tract signs indicate a compact brainstem process.
Investigation priorities
01
Serial neurological and brainstem examinationFirst step
Emergency pathwayPosterior-fossa mass with acute hydrocephalus
A child or adult with ataxia and vomiting becomes drowsy and imaging shows ventricular enlargement above a fourth-ventricle mass.
Call the age-appropriate neurosurgical, anaesthetic and critical-care teams, stabilise airway and circulation, repeat consciousness, pupils and brainstem observations, and avoid lumbar puncture.
Obtain immediate CT if not already performed and proceed to specialist MRI only when physiologically safe; neurosurgery decides whether urgent CSF diversion, tumour decompression or both are required. This shared anatomical emergency response does not erase the need for age- and entity-specific definitive oncological treatment.
Paediatric tumour pathwayStable child with a posterior-fossa mass
MRI shows a cerebellar or fourth-ventricle tumour without current cardiorespiratory deterioration.
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.
NICE NG99 brain tumours and brain metastases in over 16sPublished 11 July 2018; updated 29 January 2021. Sections 1.1–1.8 and 1.9–1.11 read for adult imaging, tissue, entity-specific management, support and late effects; explicitly over-16, not paediatric guidance.
NCI childhood medulloblastoma and CNS embryonal tumours PDQUpdated 11 April 2025. General information, staging evaluation, Table 3 and childhood treatment sections read for medulloblastoma molecular/risk and age-dependent multimodal therapy; US government peer-reviewed evidence summary, not a UK formal guideline.
NCI childhood ependymoma PDQUpdated 6 January 2025. Anatomy, clinical features, diagnostic evaluation, stage information and treatment overview read for posterior-fossa presentation, whole-neuraxis imaging and paediatric MDT management.
NCI childhood astrocytoma and other glioma PDQUpdated 14 April 2025. Treatment overview and Table 3 read for paediatric circumscribed and diffuse glioma management and late-effects context; not transferred to adults.
ENLS Intracranial Hypertension and Herniation Protocol version 6.0Neurocritical Care Society; last updated September 2024. Communication, diagnosis, Tier Zero and Tier One sections read 13 September 2026. Applies a paediatric ICP threshold and states that ICP crises require immediate treatment, CAB assessment, urgent CT when safe and emergency EVD management of imaging-confirmed acute obstructive hydrocephalus. Multi-aetiology professional consensus; quantitative adult severe-TBI targets were not transferred to this child.
Royal Children’s Hospital Melbourne clinical guideline: Lumbar punctureLast updated December 2024. Key points, assessment and contraindications read 13 September 2026. Paediatric procedural guidance identifying deteriorating or fluctuating consciousness and signs of raised ICP as absolute LP contraindications; supports the no-LP safety option alongside ENLS emergency hydrocephalus management.